In step with the progress in downsizing and performance-upgrading of electronic devices, a demand for a high capacity secondary battery has been growing further. Especially, a non-aqueous electrolyte secondary battery such as a lithium ion secondary battery which exhibits a higher energy density compared to a nickel-hydrogen battery, etc., has been drawing attention.
Although a lithium secondary battery using metallic lithium as a negative electrode material has been heretofore widely known as a typical non-aqueous electrolyte secondary battery, the same has a drawback in terms of cycle performance or safety due to deposit of dendrite-shaped lithium during battery charging. Consequently, a lithium ion secondary battery using a carbon material as a negative electrode material has been investigated especially seriously. Graphite is known as a typical carbon material, and it has been reported with respect to a lithium ion secondary battery using graphite that the electrode expansion is small and the cycle performance is superior. However, when only graphite is used as a negative electrode material, there appears a drawback that the irreversible capacity is large or the rate characteristics are inferior, and for eliminating the same an investigation on a negative electrode material combining graphite and a carbon material with low crystallinity (low degree of graphitization) has been progressed.
For example, “a hybrid graphite particle composed of granulated graphite formed by aggregating a plurality of graphite scales, and a carbonaceous layer and carbonaceous fine particles having lower crystallinity than the granulated graphite filled in internal voids and/or coated on outer surfaces of the granulated graphite, as well as a negative electrode for a lithium ion secondary battery using the same” has been proposed (refer to Patent Document 1).
Further, “a negative electrode material for a secondary battery containing: a negative electrode active material made of natural graphite; and a covering material coated with a mixture of pitch as a low crystallinity carbon material and an electroconductive material on a surface of the negative electrode active material” has been proposed (refer to Patent Document 2).
Further, “a lithium ion secondary battery material for a negative electrode composed of graphite powder particles, and hybrid particles of carbon black and carbonized pitch and having an average particle size D50 of 8 to 15 μm and a specific surface area of 15 m2/g or less” has been proposed (refer to Patent Document 3).
Furthermore, “a negative electrode active material for a lithium ion secondary battery composed of a powdery carbon material having a multiple-phase structure, which is a mixture of nearly spherical graphite particles having microspikes on a surface obtained by impregnating or coating a mixture of pitch and carbon black in/on a base material formed spherical from natural graphite, and being fired at 900° C. to 1500° C., and carbonaceous particles obtained by firing a mixture of pitch and carbon black at 900° C. to 1500° C., crushing the same, and followed by adjustment of the particle size; and has G band complex peaks near 1600 cm−1 and near 1580 cm−1, and at least one D band peak near 1380 cm−1 in a Raman spectroscopic analysis using argon laser light with a wavelength of 514.5 nm, and a lattice spacing d002 of crystal faces obtained through wide-angle X-ray diffraction from 0.335 to 0.337 nm” has been proposed (refer to Patent Document 4).